Orthodontic treatment is characterized by applying forces to individual teeth aimed at moving them to a desired position. To induce tooth movement efficiently, the forces applied on the teeth should be within a certain range: too high forces result in slow, nonexistent or pain-full tooth movement, while too low forces result in slow or nonexistent tooth movement. Thus, the orthodontic removable or fixed appliances should induce forces on the teeth within the desired range.
To apply forces on the teeth using fixed appliances, i.e. orthodontic bands or brackets and archwires, two methods are generally used: either the standard edgewise technique or the straight-wire technique.
The standard edgewise technique uses brackets with their slot at right angles relative to the tooth axis and bends made in the archwire. The type of bends (1st, 2nd, 3rd order bends) determine the forces applied to the teeth and the resulting tooth movement.
The straight-wire technique uses fully programmed brackets (varying base thickness and varying mesio-distal angulation and/or bucco-lingual inclination of slot relative to base) varying for each individual tooth and a “straight” archwire. Forces are induced by the deformation of the wire (with respect to its initial unloaded shape) applied to position and hold it into the brackets fixed on the patient's teeth. If the induced forces are within the desired range the tooth will move. As the tooth moves, the deformation of the wire reduces and the forces delivered on the teeth diminish. Once the force decreases below the threshold value for inducing tooth movement, the tooth movement will stop. Therefore during treatment different wires, having different stiffnesses (which are determined by the cross-section and the material properties [Young's modulus] of the wire) are used. Typically, at the beginning of the orthodontic treatment the wire will have a low stiffness, allowing high deformations of the wire without inducing too high forces on the teeth. The succeeding wires in the orthodontic treatment will have always a higher stiffness since forces within the specified range for tooth movement must be induced at smaller and smaller deformations.
Both types of orthodontic treatment are characterized by periodic (every four to eight weeks) meetings of the patient with the dentist/orthodontist, during which the induced tooth movements are verified and the treatment is adjusted if needed. This means that brackets may need repositioning, arch wires may need to be changed. The total treatment typically lasts one and a half up to two years, but the final treatment time is not well predictable in advance. Therefore it would be desirable to be able to simulate treatment options in advance and estimate the total treatment time in order to determine the optimal treatment.
More recently, digitalization and 3D computer planning has been introduced in the orthodontic practice. The traditional plaster models of the patient's dentition are replaced by a digital surface representation of the patient's dentition, visualized with dedicated software. These software applications typically also allow performing tooth measurements (mesio-distal width, vestibulo-lingual width, tooth height, interproximal contacts . . . ) and orthodontic analyses (symmetry, Bolton, Korkhaus, occlusal contacts . . . ). More advanced software applications allow segmentation and further repositioning of the individual teeth, positioning virtual brackets . . .
Patent application US 2005/0079468 A1 describes a method for dividing an orthodontic treatment path (from initial tooth position to desired tooth position) into clinically appropriate sub steps for repositioning the teeth of a patient. A digital finite element model is made of the patient's teeth and related mouth tissue and of the shape and material of each of a sequence of appliances to compute the actual effect of the designed appliances on the tooth movement. The resulting tooth movements are verified against clinical constraints and if needed appliances and sub steps are adapted. Finally all appliances needed for the orthodontic treatment (reposition teeth in steps as defined and verified during finite element computations) are manufactured.